instance ids

--[[ DOCUMENTATION

= module(instance) [instance]
    --> existing instance id if it exists, or new instance id

= module.new(instance) [instance]
    --> returns a new custom instance object or an existing one if one exists
        ~ .Unassociate() --> unassociate the tag with the instance
        ~ .ID --> get the instance's id

= module.GetInstanceByID(id) [string]
    --> get an instance by id. errors if id doesn't exist.

= module.UnregisterInstance(instance) [instance]
    --> unassociate a tag with an instance.

= module.UnregisterID(id) [string]
    --> unassociate a tag with an instance
]]


local HTTPService = game:GetService('HttpService')

local module = {}
module.__index = module

local registeredInstances = {}

setmetatable(module, {
    __call = function(class, instance)
        return class.new(instance).ID
    end,
})

function module.new(instance)
    if registeredInstances[instance] then
        return registeredInstances[instance]
    end

    local self = setmetatable({}, module)
    self:create(instance)
    return self
end

function module:create(instance)
    self.ID = HTTPService:GenerateGUID(false)
    registeredInstances[instance] = self

    function self.Unassociate()
        registeredInstances[instance] = nil
    end

    instance.AncestryChanged:Connect(function()
        if not instance:IsDescendantOf(game) or not instance:IsDescendantOf(nil) then
            module.UnregisterInstance(instance)
        end
    end)
end

function module.GetInstanceByID(ID)
    for instance, object in pairs(registeredInstances) do
        if object.ID == ID then
            return instance
        end
    end
    return error('No instance with id: ', ID)
end

function module.UnregisterInstance(instance)
    if registeredInstances[instance] then
        registeredInstances[instance] = nil
        return
    end
    return error('Instance doesn\'t exist: ', tostring(instance))
end

function module.UnregisterID(ID)
    for instance, object in pairs(registeredInstances) do
        if object.ID == ID then
            registeredInstances[instance] = nil
            return
        end
    end
    return error('ID doesn\'t exist: ', ID)
end

return module